Alpha probes are devices which utilize an alpha particle sensitive screen to produce scintillations in response to impingement by alpha particles, the scintillations then being usually transmitted to a photo responsive device such as a photo-multiplier tube by means of intervening light coupling devices such as lens systems, light pipes and prisms. These light coupling devices add considerably to the weight of the structure and also restrict the viewing area. Such known devices typically weigh in the range of 1 1/2 to 2 1/2 lbs., and are limited in surface detection area to scanning response surfaces generally in the range from 20 cm.sup.2 to 50 cm.sup.2. Such devices must be positioned as closely as possible to the contaminated surface being scanned because alpha particles have energies in the range from 0-5 mev and lose about 1 mev per centimeter of travel through air. When scanning irregular surfaces such as the ground, contact with grass and sticks can cause damage to the device by poking holes in the light blocking screen which shields the alpha particle responsive screen. Such holes allow light through to the photo-multiplier tube and generate false high readings. With the known devices, it is quite time consuming and cumbersome to replace the light shielding screen in the device in order to again render it operative.